The doping of electrically active elements into semiconductors is now done almost exclusively by injecting the elements into a semiconductor material by means of instruments known as ion implanters. Ion implanters create highly controlled beams of suitable ions and direct these ions to impinge upon semiconductor wafers to dope the wafers in a uniform and known manner. An essential component of an ion implanter is a device to determine the implantation dose to provide an accurate measure of the total number of nuclei implanted and the relative uniformity of that implant over the wafer area. The present state of the semiconductor technology demands that the implantation dose be uniform to 1% over the wafer area and that the absolute number of implanted nuclei be controlled to at least 2%.
A number of methods have been used for the measurement of implantation dose. A method that is presently standard in the industry is to measure the total charge delivered by the ion beam into a Faraday cage. For reliable measurements it is necessary to exclude electrons which are present in the implantation volume and it is necessary to account for the presence of neutral particles which do not register in the Faraday cage. Both these problems have been solved for Faraday cages used for determining the dose of relatively small beams, typically a few square cm in area.
Faraday cages of practical size are not so well suited to the measurement of beams with cross-sectional areas of 10 cm.sup.2 or more. For beams of this size and larger, the required large opening of the Faraday cage makes it difficult to prevent electrons from leaking in or out of the active cage volume. Moreover, the conventional design in which the length of the Faraday cage is very large compared to the size of the opening, can pose severe practical problems if space is at a premium.